Validation of selected microarray results using Northern blot analysis. RNAs from six replicates of A. salmonicida were pooled and separated on denaturating 1.2% formamide agarose gels, transferred to two membranes and tested for presence of specific RNAs using radio-labeled probes. Plus (+) indicates addition of 50 μM of the iron chelator 2,2'-dipyridyl to the cultures 15 min prior to sampling, whereas minus (−) indicates the untreated control. Numbers in the left column indicate the size of the RNA as measured from the gel, and numbers in parentheses indicate the theoretical size. The right column indicate normalized fold change values calculated from the Northern blot autoradiograms, while numbers in parentheses show the corresponding microarray fold change values.